Surface speciation of yttrium and neodymium sorbed on rutile: Interpretations using the charge distribution model

The adsorption of Y ³⁺ and Nd ³⁺ onto rutile has been evaluated over a wide range of pH (3-11) and surface loading conditions, as well as at two ionic strengths (0.03 and 0.3m), and temperatures (25 and 50°C). The experimental results reveal the same adsorption behavior for the two trivalent ions onto the rutile surface, with Nd ³⁺ first adsorbing at slightly lower pH values. The adsorption of both Y ³⁺ and Nd ³⁺ commences at pH values below the pH _znpc of rutile. The experimental results were evaluated using a charge distribution (CD) and multisite complexation (MUSIC) model, and Basic Stern layer description of the electric double layer (EDL). The coordination geometry of possible surface complexes were constrained by molecular-level information obtained from X-ray standing wave measurements and molecular dynamic (MD) simulation studies. X-ray standing wave measurements showed an inner-sphere tetradentate complex for Y ³⁺ adsorption onto the (110) rutile surface (Zhang et al., 2004b). The MD simulation studies suggest additional bidentate complexes may form. The CD values for all surface species were calculated based on a bond valence interpretation of the surface complexes identified by X-ray and MD. The calculated CD values were corrected for the effect of dipole orientation of interfacial water. At low pH, the tetradentate complex provided excellent fits to the Y ³⁺ and Nd ³⁺ experimental data. The experimental and surface complexation modeling results show a strong pH dependence, and suggest that the tetradentate surface species hydrolyze with increasing pH. Furthermore, with increased surface loading of Y ³⁺ on rutile the tetradentate binding mode was augmented by a hydrolyzed-bidentate Y ³⁺ surface complex. Collectively, the experimental and surface complexation modeling results demonstrate that solution chemistry and surface loading impacts Y ³⁺ surface speciation. The approach taken of incorporating molecular-scale information into surface complexation models (SCMs) should aid in elucidating a fundamental understating of ion-adsorption reactions.